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January,  1923 


Extension  Circular  No.  135 


N.  C.  AGRICULTURAL  EXTENSION  SERVICE 

OF  THE 

STATE  COLLEGE  AND 

STATE  DEPARTMENT  OF  AGRICULTURE  AND 

U.  S.  DEPARTMENT  OF  AGRICULTURE  COOPERATING 

B.  W.  KILGORE,  DIRECTOR 
RALEIGH 


DUSTING  OF  CABBAGE  AND  COLLARI^S 

TO  CONTROL  WORMS 


Distributed  in  Furtherance  of  the  Acts  of  Congress  of  May  8  and  June  30, 1914 


0 

* 

i  t 


JOINT  COMMITTEE  FOR  AGRICULTURAL  WORK 


Governor  Cameron  Morrison . Raleigh  - 

W.  C.  Riddick . Raleigh 

« 

W.  A.  Graham . Raleigh 

R.  L.  Woodard . ......Pamlico 

Clarence  Poe  . Raleigh 

A.  T.  McCallum . Red  Springs 

C.  C.  Wright . Hunting  Creek 

0.  L.  Clark . Clarkton 

T.  T.  Thorne . Rocky  Mount 

C.  W.  Gold . Greensboro 

J.  P.  Diggs . Rockingham 


DUSTING  OF  CABBAGE  AND  COLLARDS  TO  CONTROL  WORMS 

By  R.  W.  Leiby  and  S.  C.  Clapp 


In  this  State  the  several  kinds  of  worms  which  attack  cabbage  and  col- 
lards*  are  responsible  for  greater  injury  in  the  aggregate  than  any  other  pest 
of  these  plants.  In  the  western  part  of  the  State  it  is  not  unusual  to  see 
many  fields  of  cabbage  in  which  every  plant  shows  injury  by  worms  to  some 
extent.  The  cabbage  grower  sells  only  the  head  of  the  plant,  and  the  con¬ 
sumer  sees  little,  therefore,  of  the  actual  loss.  At  harvest,  however,  the 
grower  is  frequently  forced  to  discard  numerous  incompletely  headed  plants. 
Such  plants  fail  to  head  properly  not  only  because  of  the  direct  feeding  of 
the  worms  upon  the  outer  leaves,  but  also  because  such  plants  are  damaged 
through  loss  of  sap,  and  the  drying  out  of  the  heads  and  stems  that  are 
occasionally  eaten  into.  In  this  connection  we  have  observed  that  cabbages 
will  outgrow  limited  injury  by  worms  in  a  favorable  (usually  wet)  season, 
whereas  in  a  rather  dry  season  the  continued  feeding  of  the  worms,  together 
with  the  slow  growth  of  the  plant,  will  often  stunt  it,  and  the  plant  will  fail 
to  head  or  head  only  partially.  In  any  event  the  feeding  of  the  worms  is  a 
detrimental  factor  to  the  growing  of  good  cabbage. 

In  the  eastern  part  of  the  State,  where  early  spring  cabbage  is  commonly 
grown,  the  injury  by  worms  at  this  time  is  not  so  serious.  In  the  eastern 
part  of  the  State,  however,  collards  are  grown  in  late  summer  and  early  fall; 
and  their  leaves  are  often  riddled  by  worms,  to  the  extent  that  plants  will  be 
matured  only  in  a  very  favorable  growing  season  without  some  treatment 
for  worms. 

It  should  be  understood  that  there  are  at  least  five  different  kinds  of 
worms  that  feed  upon  cabbage  and  collards  in  this  State,  but  all  of  them  are 
the  larvae  (or  caterpillar  stage)  of  moths  or  butterflies.  The  two  commonest 
and  most  destructive  kinds  of  worms  are  the  Cabbage  Looper  (Autographa 
hrassicw  Riley),  and  the  Imported  Cabbage  worm  (Pontia  rapw  Linn). 

This  circular  is  prepared  to  enable  cabbage  and  collard  growers  to  dis¬ 
tinguish  the  kinds  of  worms  which  are  responsible  for  most  of  the  injury 
to  these  plants,  and  to  control  their  injuries  by  dusting  with  a  mixture  of 
one  pound  of  dry  powdered  lead-arsenate  and  six  pounds  of  dry  powdered 
(air-slaked)  lime.  The  control  measures  here  recommended  are  based  upon 
six  extensive  experiments  conducted  on  cabbage  and  collards  in  this  State, 
at  Raleigh,  and  the  branch  experiment  station  at  Swannanoa,  during  the 
years  1917  to  1920,  inclusive. 


THE  CABBAGE  LOOPER 


The  cabbage  looper  is  principally  a  pest  on  cabbage  and  collards,  although 
it  feeds  upon  kale,  turnip,  lettuce  and  celery.  The  looping  habit  of  this 
worm  and  the  arched  position  of  its  body  on  the  leaf,  readily  distinguish  it 
from  the  imported  cabbage  worm,  which  lies  flat  upon  and  close  to  the  leaf. 
The  cabbage  looper  is  found  very  commonly,  in  fact  nearly  always,  on  the 
under  side  of  the  cabbage  leaf;  while  the  imported  cabbage  worm  is  found 
usually  on  the  upper  side  of  the  leaf. 


*  For  the  benefit  of  readers  in  the  western  part  of  the  State  and  others,  it  may  be  said 
that  the  “collard”  is  a  cabbage-like  vegetable  with  rather  long  stem,  which  does  not  form 
a  head  By  many  it  is  preferred  to  the  cabbage.  It  is  more  especially  a  fall  and  winter 
vegetable.  It  is  grown  for  home  use  throughout  the  warmer  part  of  the  S.ate  and  corre¬ 
sponding  parts  of  other  Southern  states. 


4 


C.  Agricultural  Extension  Service 


This  insect  passes  the  winter  in  the  pupa  or  resting  stage  in  a  flimsy  silken 
cocoon.  The  very  small  rounded  white  eggs  are  deposited  by  the  moths  in 
spring  on  both  sides  of  the  leaves,  but  more  frequently  on  the  upper  side. 
The  eggs  hatch  in  six  to  nine  days;  in  the  fall  the  egg  stage  covers  a  period 
of  seven  or  eight  days.  The  young  larvae  or  worms  that  hatch  from  the  eggs 
are  at  first  pale  green  in  color,  but  as  they  become  grown,  the  larvae  are 
marked  with  longitudinal  white  stripes.  The  full  grown  worm  measures 
1%  to  iy2  inches  in  length.  About  26  days  are  required  for  the  larva  to 
become  fully  grown. 


Fig.  1.  Dusting  cabbage  with  a  hand  dust-gun.  Swannanoa,  1918. 


After  the  worm  is  grown  it  spins  a  flimsy  cocoon  of  white  silk  in  which  it 
changes  to  the  pupa  stage.  This  pupa  is  first  pale  green  in  color,  but  as 
development  proceeds,  and  just  before  the  moth  emerges  from  it,  the  cocoon 
is  dark  brown  or  blackish,  and  it  can  be  readily  seen  through  the  flimsy 
cocoon.  The  cocoon  is  spun  upon  the  upper  or  underside  of  the  leaf,  but 
usually  in  a  more  or  less  secluded*  place.  The  pupal  stage  covers  about 
twelve  days  in  fall,  and  less  time  in  summer. 

The  moths  are  dark  gray  to  black  in  general  color,  and  have  two  promi¬ 
nent  silver  spots  (one  of  which  is  U-shaped),  located  close  to  each  other  or 
in  contact,  near  the  center  of  each  forewing.  The  moths  are  good  fliers,  and 


Dusting  Cabbages  and  Collards 


5 


thGy  are  very  active  during  the  late  afternoon  and  early  evening,  when  they 
may  be  commonly  collected  on  Zinnia  flowers  from  which  they  gather  nectar. 
With  the  nectar  of  Zinnia  flowers  as  food,  we  have  kept  the  moths  alive  in 
outdoor  cages  for  two  weeks,  during  which  time  we  have  observed  that  the 
females  will  deposit  from  150  to  200  eggs. 

The  larva  of  this  insect  is  not  often  killed  by  insect  parasites,  but  a  dis¬ 
ease  frequently  spreads  among  the  larvae  in  fall  and  kills  great  numbers  of 
them.  Diseased  larv«  become  inactive,  turn  a  sickly  yellowish  color,  and 
soon  die.  After  death  the  skin  of  the  diseased  black  larvae  breaks  open 
and  the  black  content  smears  out  over  the  leaves. 

We  have  found  that  in  late  fall  this  insect  requires  from  flve  to  seven 
weeks  to  develop  from  the  egg,  through  the  larva  and  pupal  stages,  to  the 
moth  stage.  This  period  is  considerably  shortened  in  summer,  however, 
and  there  are  probably  at  least  four  generations  each  year. 

THE  IMPORTED  CABBAGE  WORM 

A  fully  grown  imported  cabbage  worm  is  velvety  green  in  color  and  marked 
with  a  greenish  yellow  stripe  down  the  back,  and  a  more  or  less  broken  one 
on  each  side.  It  measures  about  an  inch  in  length.  The  larvae  of  this 
species  are  frequently  found  somewhat  deep  within  the  cabbage  head,  at  its 
base,  where  they  are  difficult  to  poison.  They  regularly  lie  flat  on  the  sur¬ 
face  of  the  leaf. 

This  insect  passes  the  winter  as  a  pupa  or  chrysalis,  but  the  chrysalis  is 
not  encased  in  a  silken  cocoon  like  that  of  the  cabbage  looper.  The  adult  of 
the  imported  cabbage  worm  is  a  white  butterfly — the  small  white  butterflies 
seen  flitting  about  commonly  over  cabbage  and  collard  plants  during  the  sum¬ 
mer,  are  almost  invariably  the  parent  forms  of  the  imported  cabbage  worm. 
These  butterflies  lay  orange-colored  eggs  on  the  underside  of  cabbage  or 
collard  leaves.  The  young  worms,  upon  hatching  from  the  eggs,  first  skele¬ 
tonize  the  leaves,  but  as  they  become  grown,  all  of  the  leaf  except  the  larger 
veins,  are  often  eaten.  When  the  worm  is  fully  grown  it  seeks  a  secluded 
place,  usually  on  the  plant,  where  it  spins  a  layer  of  silk  over  the  place 
selected,  and  prepares  to  change  to  the  pupa  or  chrysalis  stage.  After  spin¬ 
ning  the  layer  of  silk  over  the  chosen  plaee,  the  worm  spins  a  silken  cord 
over  the  front  part  of  the  body,  which  serves  to  fasten  it  securely  to  the  place 
of  pupation.  The  skin  of  the  worm  is  then  shed  and  the  chrysalis  appears; 
the  insect  being  still  securely  held  in  place  by  the  previously  spun  silken 
cord.  In  a  week  or  ten  days  after  pupation  the  butterfly  emerges  from  the 
chrysalis.  There  are  five  or  six  generations  of  this  insect  developed  each 
year. 

CONTROL  OF  CABBAGE  AND  COLLARD  WORMS 

The  experiments  in  the  control  of  cabbage  worms,  upon  which  we  base 
the  recommendations  in  this  circular,  were  carried  out  in  1917  and  1918 
on  cabbage  and  collards  at  Raleigh,  and  in  the  years  1917  to  1920  on 
cabbage  at  Swannanoa.  At  Raleigh,  in  1917  and  1918,  four  plats  each  of 
cabbage  and  collards  were  selected.  One  plat  was  dusted  weekly  with  a 
mixture  of  one  part  arsenate  of  lead  and  six  parts  of  lime;  one  plat  was 
dusted  every  two  weeks  with  this  same  mixture;  one  plat  had  the  worms 
removed  by  handpicking;  and  the  fourth  plat  was  left  untreated  to  serve  as 
a  check. 

In  1917,  1918,  and  1919,  this  work  was  duplicated  on  late  cabbage  at  Swan¬ 
nanoa,  except  that  an  additional  plat  was  dusted  every  three  weeks.  The 
cabbage  grown  was  a  standard  variety  that  was  planted  in  July  and  har¬ 
vested  in  October.  In  1920  the  investigations  were  modified  to  the  extent 


6 


N.  C.  Agricultural  Extension  Service 


that  the  dusting  was  done  at  intervals  of  ten  days  only,  and  during  the 
time  that  the  worms  were  numerous;  but  the  dusting  was  done  on  sixteen 
different  varieties  of  cabbage.  These  experiments  have  led  to  the  following 
conclusions. 

The  7nate7'ial  to  use. — We  have  found  that  one  pound  of  dry  powdered  lead 
arsenate  mixed  thoroughly  with  six  pounds  of  powdered  (air-slaked)  lime 
will  poison  the  worms  quickly — the  older  ones  in  two  or  three  days,  the 
younger  ones  as  soon  as  they  feed  a  little  on  the  plants.  The  more  recent 
insecticide,  calcium  arsenate,  when  mixed  with  lime  in  the  same  proportions, 
would  probably  be  even  more  effective,  although  this  insecticide  was  not 
used  in  our  experiments. 

When  and  how  often  to  apply  the  mixture. — It  seems  best  that  the  dust  be 
applied  when  the  plants  are  moist,  for  the  reason  that  the  dust  adheres 
better.  This  can  be  done  early  in  the  morning  when  there  is  dew  on  the 
plants  or  immediately  after  a  shower. 


Fig.  2.  Undusted  cabbage  plants  showing  severe  injury  caused  by  the  feeding  of  cabbage 
worms.  Swannanoa,  1918. 

The  first  application  is  necessary  on  cabbages  before  they  begin  to  head, 
and  on  collards  when  eight  to  a  dozen  leaves  are  unfolded;  for  at  this  time 
worms  are  nearly  always  present,  at  least  on  the  late  cabbage.  Since  the 
dust  is  readily  washed  off  the  leaves,  and  since  the  adults  of  the  worms  are 
continually  depositing  eggs  throughout  the  growing  season  (at  least  in  late 
summer),  it  is  necessary  to  repeat  the  application  every  ten  days.  When 
the  cabbages  are  partly  headed  and  the  collards  well  grown,  the  injuries  of 
the  worms  are  not  so  serious,  since  the  plants,  if  properly  cultivated  and 
fertilized,  will  outgrow,  to  a  limited  extent,  the  feeding  of  the  worms.  We 
have  observed  that  this  occurs  particularly  in  a  favorable  growing  season 
when  there  is  sufficient  rainfall.  At  such  times,  and  under  such  conditions, 
the  grower  can  afford  to  omit  an  application  of  dust,  but  then  only,  when  he 
has  applied  the  dust  regularly  two  or  three  times  while  the  plants  were 
young.  We  find,  however,  that  four  to  five  applications  are  advisable  dur¬ 
ing  the  season  on  late  cabbage,  as  it  is  grown  in  the  western  part  of  the 
State,  the  first  application  being  made  about  August  1st  and  the  last  some 
time  in  late  September.  In  general,  the  dusting  should  be  done  every  ten 
days  for  a  month  shortly  after  the  cabbage  is  planted,  when  one  or  two 
applications  may,  sometimes,  be  omitted  (that  is,  in  a  favorable  growing 
season)  and  then  one  or  two  subsequent  applications  made.  To  secure  good 
control  of  the  worms  the  grower  must  adhere  to  a  regular  schedule  of 
dusting,  and  he  should  regularly  and  carefully  examine  the  plants  to  ascer¬ 
tain  whether  the  worms  are  numerous. 


Dusting  Cabbages  and  Collards  7 

Handpicking  the  worms  in  the  case  of  large  plantings  of  cabbage  and 
collards,  according  to  our  records,  is  not  profitable,  although  this  is  at 
present  the  common  control  method  practiced  by  farmers  on  garden  plantings. 
Handpicking  is  laborious  and  apt  to  be  carelessly  done  by  the  average  farm 
laborer. 

How  to  apply  the  dust. — The  mixture  is  best  applied  by  using  a  hand 
dust-gun.  The  type  shown  in  Fig.  1  is  not  as  satisfactory  as  the  fan-geared 
hand  duster,  which  is  strapped  to  the  body  and  operated  by  a  hand  crank, 
and  which  is  now  commonly  used  in  dusting  cotton  and  tobacco.  A  good 
type  of  hand  duster  can  be  procured  for  about  twelve  to  sixteen  dollars.  The 
method  of  applying  the  dust  by  means  of  shaking  it  through  a  sack  is  not 
satisfactory,  because  the  dust  cannot  be  directed  to  the  base  of  the  plant, 
where  the  worms,  especially  the  large  destructive  ones,  are  feeding. 


Fig.  3.  Dusted  cabbage  plants.  The  plat  in  which  this  photograph  was  taken  was 
adjacent  to  the  undusted  plat,  a  part  of  which  is  shown  in  Figure  2.  Swannanoa,  1918. 


From  18  to  20  pounds  of  the  mixture  are  necessary  for  a  single  application 
of  the  dust  on  an  acre  of  well-grown  cabbage.  Only  one-seventh  of  this  mix¬ 
ture  is  lead  arsenate,  and,  therefore,  between  two  and  three  pounds  of  the 
poison  are  used  per  acre  for  each  application. 

Is  there  danger  of  poisoning  in  eating  eahhage  or  collards  ivhen  dusted  as 
recommended? — In  1911  a  bulletin,  entitled  “Insect  Enemies  of  Cabbage,” 
was  published  by  the  North  Carolina  Department  of  Agriculture,  in  which 
Professor  Franklin  Sherman,  its  author,  advised  the  dusting  of  cabbage 
with  a  mixture  of  one  ounce  of  dry  paris  green  and  one  pound  of  dry,  air- 
slaked  lime,  to  control  cabbage  worms.  This  recommendation  has  been 
freely  followed,  and  we  have  yet  to  hear  of  any  fatal  results  from  the  eating 
of  cabbage  when  so  dusted.  Similar  recommendations  are  made  for  the  con¬ 
trol  of  cabbage  worms  in  other  states;  nowhere  is  there  a  record  of  any 
death  caused  by  eating  cabbage  that  was  dusted  to  control  the  worms. 
Moreover,  cabbage  treated  for  worms  according  to  recommendations,  has  been 
repeatedly  analyzed  for  arsenate  within  a  reasonable  time  after  application  of 
the  poison,  and  the  usual  finding  is  only  a  “trace”;  and  not  enough  to  poison 
a  person  under  any  circumstances.  The  truth  of  the  matter  is,  that  a  good 
percentage  of  cabbage  is  treated  for  worms  and  being  constantly  consumed 
by  the  public  without  ill  effects. 

It  is  assumed,  of  course,  that  the  cabbage  grower  will  exercise  ordinary 
good  sense  and  judgment  in  dusting  his  plants.  It  is  never  necessary  to 
cover  a  leaf  or  plant  with  a  thick  coating  of  the  dust.  A  light,  uniform 


8 


IST.  C.  Ageicultural  Extension  Service 


application,  is  not  only  desirable  but  more  effective  on  the  worms.  It  is 
further  assumed  that  growers  will  not  treat  the  cabbage  just  before  it  is  to 
be  cut;  this  is  not  advised,  nor  is  it  necessary.  Furthermore,  in  the  case  of 
cabbage,  the  plant  grows  and  heads  from  the  inside,  and  it  is  practically  im¬ 
possible  to  apply  the  dust  to  the  part  of  the  plant  usually  eaten. 

Likewise,  there  is  no  danger  in  eating  collards  when  dusted  as  herein 
recommended.  Our  experience  shows  that  over  a  period  of  two  or  three 
weeks  there  is  not  sufficient  poison  remaining  on  the  leaves  under  the  usual 
growing  conditions  to  poison  a  cabbage  worm;  hence  we  recommend  that 
the  applications  be  made  every  ten  days.  It  is  obvious  then  that  one  who 
consumes  a  part  of  a  collard  will  not  be  poisoned  if  there  is  not  sufficient 
poison  present  in  the  first  place  to  kill  a  worm. 

Finally,  we  are  certainly  safe  in  the  conclusion  that  applications  of 
arsenate  of  lead  and  lime,  as  recommended,  can  be  made  at  any  time  while 
the  cabbage  and  collards  are  growing,  without  any  injury  or  danger  what¬ 
ever  to  the  persons  who  finally  eat  the  plants.  This  conclusion  is  particu¬ 
larly  safe  if,  in  the  case  of  cabbage,  the  outer  rough  leaves  are  discarded 
and  if  the  collard  leaves  are  washed  prior  to  consumption,  as  is  usually  done 
in  both  cases. 


